We analyzed monthly survival rates, persistence of young, reproduction, recruitment of young, immigration rates, body mass, and sex ratio for fluctuating sympatric populations of Microtus ochrogaster and M. pennsylvanicus in alfalfa, bluegrass and tallgrass over 25 years. M. ochrogaster underwent 13 population cycles in alfalfa, 12 in bluegrass and five in tallgrass. M. pennsylvanicus displayed five cycles in alfalfa, nine in bluegrass and was acyclic in tallgrass. Among the demographic variables considered, only increased survival and persistence of young were associated consistently with the increase phase of population cycles in both species. Survival rates, persistence of young and reproduction of M. ochrogaster were lower during the decline than during the increase phase in alfalfa and bluegrass, but not in tallgrass. There were no demographic differences between the decline and increase phases for M. pennsylvanicus in either alfalfa or bluegrass. Most differences in demographic variables within phases among habitats involved survival and explained among-habitat demographic differences of each species. Differences in demographic variables within phases between species also mainly involved survival; these differences were consistent with differences in demography of the two species in alfalfa. Overall, our data indicate that changes in survival were most important in determining patterns of fluctuation in our study populations.